I. Field of the Invention
The present invention relates generally to mobile telephone systems, and more particularly to methods and apparatus for controlling the transmission power of mobile telephones in code division multiple access (CDMA) mobile telephone systems.
II. Description of the Related Art
In code division multiple access (CDMA) mobile telephone systems, mobile telephones in the system using CDMA spread-spectrum principles transmit their signals over the air to base stations in the system. Details of a preferred CDMA system are set forth in U.S. Pat. No. 4,901,307 for an invention entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters" and U.S. Pat. No. 5,103,459 for an invention entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System", assigned to the same assignee as is the present invention and incorporated herein by reference. CDMA transmissions for use in cellular communication has been standardized by the Telecommunications Industry Association publication no. TIA/EIA/IS-95-A, entitled "A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereinafter "IS-95-A").
It is important that when multiple mobile telephones transmit their signals to a base station, the various signals which are received by a base station arrive at the base station with amplitudes that are approximately equal to each other. This is necessary to ensure that the quality of communication with each mobile station is the same. More particularly, in a CDMA communication system, each user's signal is noise to every other user, and providing for roughly equal mobile station signal power at the base station insures that no single mobile station is disproportionately contributing to the interference of other mobile stations.
To regulate the received power level of the transmitted signals from mobile telephones, a base station that is communicating with multiple mobile telephones periodically transmits, in a power control subchannel of a traffic channel, a distinct sequence of power control signals to each telephone. In current CDMA systems, a base station transmits power control signals to each mobile telephone by inserting (at a rate of 800 times per second) a single power control bit into the communication signal (referred to herein as the "traffic" signal) being transmitted by the base station. For purposes of disclosure, the power control bit will be referred to in the singular, although it is to be understood that the present discussion applies equally to instances wherein two successive power control bits are inserted into the traffic signal. The power control bit is either a logical "one" or a logical "zero", with the logic states respectively representing commands to the mobile telephone to increase or decrease the amplitude of its transmitted power by a predetermined unit amount of power. Details of such a system are set forth in U.S. Pat. No. 5,506,109 for an invention entitled "Method and Apparatus for Controlling Power in a CDMA Cellular Mobile Telephone System", assigned to the same assignee as is the present invention and incorporated herein by reference.
Signals from a base station to a mobile telephone might arrive at the mobile telephone over more than one transmission path, a phenomenon referred to as "multipath". More specifically, a base station signal might propagate over a relatively direct path to a mobile telephone, and it also might propagate over an indirect path, being reflected by buildings and the like. Each version of the signal received over a particular signal path is colloquially referred to as a "finger". To accommodate processing of multipath fingers, diversity receivers have been provided that are capable of independently demodulating each of the multipath signals and combining the demodulated signals to provide an improved estimate of the information contained in the multipath signal. A preferred diversity receiver is disclosed in U.S. Pat. No. 5,109,390 for an invention entitled "Diversity Receiver in a CDMA Cellular Telephone System", assigned to the same assignee as the present invention and incorporated herein by reference.
Accordingly, to process the various time-staggered versions of a signal, a CDMA mobile telephone includes a type of diversity receiver known as a rake receiver in which a searcher assigns a received finger to a respective one of a plurality of CDMA demodulators in a first set of demodulators, with the first set being assigned to fingers of a first base station. It is possible to know which fingers correspond to which base station, because the encoding systems (i.e., the pseudorandom noise (PN) generators) of the base stations are staggered from each other by a known and relatively large amount, generally represented as a number of PN "chips". Likewise, when the signal is received from a second base station in addition to the first, a searcher assigns received fingers from the second base station to a second set of demodulators in the rake receiver.
Moreover, when a mobile telephone is located roughly between two base stations, communication with the mobile telephone is "handed off" from one base station to the other base station during the mobile telephone's transit between the two base stations. Handoffs come in two varieties. In a hard handoff, the mobile station breaks of communication with the base station cell being exited before establishing a communication link with the base station cell being entered. In contrast, in a soft handoff, the mobile station establishes communication with the cell being entered before terminating communication with the cell being exited. The preferred method for performing soft handoff is described in detail in U.S. Pat. No. 5,101,501 for an invention entitled "Method and System for Performing a Soft Handoff in Communications in a CDMA Cellular Communication System", assigned to the same assignee as the present invention and incorporated herein by reference.
During a soft hand off procedure, there is a time period in which both base stations broadcast the same signal to the mobile telephone. Thus, not only can a mobile telephone receive a signal from a single base station over multiple paths, but it can receive the same signal from two base stations. Consequently, the mobile telephone can receive several versions of the same signal.
Accordingly, a mobile station that is in soft handoff with two or more base stations receives the same data signal over traffic channels from each base station, but receives independent power control commands in the power control subchannels from the respective base stations. Consequently, it can happen that one base station might command the mobile telephone to increase its transmitted power, whereas another base station might simultaneously command the mobile telephone to decrease its transmitted power. A mobile station conforming to the IS/95-A standard is required to decrease its power if any base station issues a "decrease" power control command, regardless of what the other base stations might command. This convention is colloquially referred to in logic terms as the "OR of the downs" rule.
There are two situations in which a mobile station can receive multiple signal components that contain the same power control command. The first is when a mobile station receives two or more multipath signal components from the same base station, in which case it is clear that the power control bits contained in each component are identical. Additionally, when sectorized base stations are used, and a mobile station receives signals from two or more sectors of the same base station (a situation commonly referred to as a "softer" handoff), identical power control bits are transmitted from each sector. In either case, the mobile station combines all received power control signals that originate from the same base station to yield a single overall estimate of the power control command sent from that base station. This combining of power control signals increases the reliability of the estimate of the command, and it is performed before applying the "OR of the downs" rule to independent power control bits that have been received from different base stations.
When power control signals from multiple base stations have been received by a mobile station, if the signal from one of the base stations is insufficient for the mobile station to make a reliable decision for the command represented by the respective power control signal, the mobile station should exclude the base station's power control command from the "OR of the downs" rule. Otherwise, the decision rendered from the weak signal, while being consequently unreliable, would nevertheless have influence in controlling the mobile station's transmitted power. Therefore, a mobile station should have a mechanism for excluding, from consideration in the "OR of the downs" rule, power command decisions that are based on weak power control signals.
Current implementations of mobile stations use a "finger lock" mechanism to exclude unreliable power control signals from the decision logic. On each finger of the RAKE receiver, the demodulated signal energy is compared to a threshold which represents a minimum energy required to reliably use power control data from that finger. If the demodulated signal energy exceeds the minimum threshold, the finger is said to be "in lock". Otherwise, the finger is said to be "out of lock". When the power control signals received on distinct fingers but originating from the same base station are combined, as described above, only those fingers which are in lock are considered. Accordingly, a base station is excluded from the "OR of the downs" rule at any moment in which there is no demodulating finger currently in lock with a signal from that base station.
The present invention recognizes, however, that although weak signals should be excluded from the final decision used in the "OR of the downs" rule, the decision to exclude these weak signals can be deferred until after they have been combined with other signals from the same base station to thereby potentially render a reliable input to the "OR of the downs" logic. Thus, in the present invention, all fingers receiving signals from a base station will be considered to form a combined estimate of the power control command from the base station. Then, the aggregate energy from all of the fingers is compared to a minimum threshold to determine whether the aggregate energy possess sufficient energy to be included in the "OR of the downs" rule.
Accordingly, it is an object of the present invention to provide a system for controlling the transmitted power of a mobile telephone. A further object of the present invention is to provide a system for controlling the transmitted power of a mobile telephone which uses power control bits from rake receiver demodulators that are not necessarily locked onto their respective fingers. Still another object of the present invention is to provide a system for controlling the transmitted power of a mobile telephone that is easy to use and cost-effective to manufacture and implement.